In common practice, synthetics yarns and fabrics made out of those yarns, such as polyesters and polyamides, are dyed with commercially available dispersed dyes at high temperatures, in jet dyeing machines.
Jet dyeing is a batch process, leading to the penetration of dye molecules deep into yarns and fabrics, because of the high pressure and temperature in a closed vessel. This deep penetration of the dye, makes this process not able to provide the “ring effect” to the yarns (or “surface dyeing” to the fabrics), which is a crucial feature for denim industry. As it is known, ring effect dyeing (i.e., perimeter dyeing), is defined as a dyeing that does not completely penetrate into the yarn (or into the fabric), but only on its surface, thus an external dyeing is achieved.
The currently available indigo dyeing process is continuous, but it can only be used for cotton and cotton rich yarns and fabrics and is not applicable to synthetic materials. This inconvenience is due to the fact that synthetic yarns and fabrics have hydrophobic surfaces, which make them difficult to be dyed with waterborne dyeing solutions such as, indeed, indigo dye solutions.
Thus, conventional indigo cannot be used as a dye for synthetic yarns and fabrics using the current infrastructure and technology.
Therefore, there is a need to find compounds and processes that allow to dye synthetic textiles showing a ring effect, using conventional continuous indigo dyeing industrial plants.